someone put it best in that the reason people do this (and a lot of people do this, don't kid yourself) isn't necessarily because they felt they knew the person, but rather through their art (music, acting, whatehaveyou) they feel they knew themselves. it's an attachment because of how their art has made us feel and examine ourselves. and it's completely human to do so.

you don't have that reaction right now because lemmy kilmister or david bowie or alan rickman or whoever else has died didn't impact you in a significant way. or maybe no artist has. but other people had that impact, and that's why they mourn.